Organic light-emitting devices (OLEDs) are electroluminescent devices which contain one or more organic compounds arranged as thin films between two electrodes, one of which is substantially transparent. They are typically deposited in rows and columns onto a flat carrier by evaporation in a vacuum, casting from a solvent or a variety of “printing” processes. The resulting matrix of pixels can display an image by emitting light of different colors or can be arranged to uniformly emit colored or white light for use as an illumination device.
OLEDs can be used in television screens, computer displays, portable system screens, advertising, information and as indicator lamps. OLEDs can also be used in light sources for general space illumination, and large-area light-emitting elements.
A significant benefit of OLEDs relates to their efficiency in turning electrical power into light. For example, green OLEDs have been demonstrated at an efficiency of 130 lumens per watt. Furthermore, OLED displays are preferred over traditional liquid crystal displays (LCDs) because OLEDs do not require a backlight to function. Thus they draw less power and, therefore, when powered from a battery, can operate longer on the same charge. Because there is no need for a backlight, polarizers or color filter array, an OLED display can also be much thinner than a LCD panel and may also be cheaper to manufacture.
Bernanose and co-workers first produced electroluminescence in organic materials in the early 1950s by applying a high-voltage alternating current (AC) field to crystalline thin films of acridine orange and quinacrine. In 1960, researchers at Dow Chemical developed AC-driven electroluminescent cells using doped anthracene. Such materials are generally known as “small molecules.”
The first low voltage thin film OLED was invented at Eastman Kodak by Dr. Ching Tang and Steven Van Slyke in the 1980s. This device used a two-layer structure with separate hole transporting and electron transporting layers such that recombination and light emission occurred at the interface between the two organic layers. This resulted in a reduction in operating voltage and improvements in efficiency, and started the current era of OLED research and device production. Later, this concept was adapted for use with polymers as reported for a green-light-emitting polymer in the Burroughs et al. 1990 paper in the journal Nature. Both small molecule and polymer OLEDs have now been widely studied and demonstrated in commercial prototypes.
More recently, the use of phosphorescent instead of fluorescent light emitting materials enabled much higher OLED efficiency (e.g. “Highly efficient phosphorescent emission from organic electroluminescent devices.” M. A. Baldo, D. F. O'Brien, Y. You, A. Shoustikov, S. Sibley, M. E. Thompson and S. R. Forrest Nature 395, 151-154 (10 Sep. 1998). Typically, organic phosphors are doped into a conductive host matrix and emission results from energy transfer from the host to the triplet state of the phosphor. Development of efficient blue OLEDs based on this technology, however, has been particularly challenging because the host material must exhibit triplet level emission at ≦470 nm to achieve efficient energy transfer and, furthermore, this high triplet exciton energy must be achieved without sacrificing the good charge transporting properties of the host layer. Current host materials do not meet these requirements, because there is a tradeoff between increasing the bandgap of the material to increase emission energies and decreasing the π-aromatic system, which may adversely affect charge transport properties. Deeper blue phosphors have only been demonstrated by using substantially insulating, wide bandgap host materials, in which charge transport occurs via hopping between adjacent dopant molecules. This leads to a high voltage, particularly at the high current densities required for bright lighting applications, and therefore less efficient devices.
An alternative route to design host molecules for efficient blue phosphorescence at low voltages is to use a point of saturation, such as a phosphoryl group, to link small, high triplet energy molecular fragments into a larger molecule without extending the conjugation length of the fragments and thereby lowering the triplet exciton energy of the larger molecule. This is important because very small molecules tend to be volatile and have poor film-forming properties so it is desirable to reproduce their photophysical properties in a larger, stable molecule. For example, aromatic diphosphine oxides are stable compounds which exhibit electroluminescence in the ultraviolet spectral region (335 nm for one example already published as P. E. Burrows, A. Padmaperuma, and L. S. Sapochak, P. Djurovich and M. E. Thompson “Ultraviolet Electroluminescence and Blue-Green Phosphorescence using an Organic Diphosphine Oxide Charge Transporting Layer.” Appl. Phys. Lett. 88, 183503 (2006)). Thus, it is possible to achieve a high triplet exciton energy without sacrificing the aromatic backbone of the molecule, which makes these materials excellent hosts for high efficiency blue phosphors, as well as longer wavelength OLEDs. Furthermore, the inductive influence of the phosphoryl group gives rise to good electron transport at low voltages. Unfortunately, however, molecules based on phosphoryl compounds have not shown good hole transporting properties. This leads to unequal densities of electrons and holes in the organic layer and, consequently, sub-optimal efficiency. Indeed, it has been demonstrated in the literature that most of the holes that penetrate into the recombination zone are actually transported via hopping conduction on the phosphorescent dopant rather than the host itself.
The low injection and transport efficiency of holes in organic phosphine oxide compounds is due to the stabilization of the highest occupied molecular orbital (HOMO) by the phosphoryl groups, leading to a very low HOMO energy and a concomitantly high injection barrier for holes from the hole transporting layer into the phosphine oxide host layer. It is therefore necessary to seek further improvements to the phosphine oxide materials which permit the HOMO of the host layer to be matched to that of the hole transport layer while not raising the LUMO such that a barrier is introduced for electron injection.
The need for more efficient use of electricity has spurred large and ongoing investments into research and development aimed at improving OLEDs. Those having ordinary skill in the art are continually seeking to improve the efficiency of OLED devices, and those having skill in the art recognize that any inventions or discoveries that increase the efficiency of OLEDs represent an advance in the state of the art. It is known to those skilled in the art that phosphoryl-containing organic molecules can be used to make thin films with good electron transporting properties and high triplet exciton energies but such materials invariably show poor hole transport properties and, indeed, they have even been proposed as hole blocking layers. To achieve the maximum possible efficiency in an OLED, however, it is important to ensure an equal density of holes and electrons in the recombination layer which requires good electron transport and hole transport properties. The present invention provides this improvement, and consequently improved efficiency in OLED devices, and thus represents an advance in the state of the art.